1. Field of the Invention
The present invention relates to a hole machining device for drilling a workpiece, having some structural forms such as H-beam and U-beam. More particularly, the hole machining device includes a clamp apparatus for clamping the workpiece at a predetermined drilling position.
2. Description of the Related Art
A clamp apparatus in conventional hole machining devices includes a stationary clamp, which is disposed at the drilling position and which contacts the outer peripheral surface of a first flange of a workpiece facing the stationary clamp. A movable clamp is provided, and contacts the outer peripheral surface of a second flange of the workpiece, facing the movable clamp apparatus. The movable clamp is driven toward the stationary clamp. Therefore, both the first and second flanges of the workpiece are clamped by means of the two clamps.
However, in the conventional clamp apparatus, the first and second flanges are clamped with a significant lateral large force, in order to securely clamp the workpiece between the stationary clamp and the movable clamp. Thus, if the workpiece such as a structural beam, does not have an even surface, the workpiece cannot be securely clamped. Further, during clamping, the workpiece is caused to bend, and its surface is further twisted. Consequently, the drilling of the workpiece cannot be achieved accurately at the precise location.
Nowadays, large structural beams such as H-beams are used in the high-rise buildings. H-beam is constructed in such a way that a web is welded between a pair of relatively large flanges. When such an H-beam is clamped at both outer peripheral surfaces of the first and second flanges by the conventional clamp apparatus the web tends to warp. Therefore, it would be difficult to securely clamp the warped large H-beam, and the drilling operation cannot be achieved accurately.